The year is 2020 and the gasoline tax is history. In its place you get a monthly tax bill based on each mile you drove — tracked by a Global Positioning System device in your car and uploaded to a billing center.

What once was science fiction is being field-tested by the University of Iowa to iron out the wrinkles should a by-the-mile road tax ever be enacted.

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Besides the technological advances making such a tax possible, the idea is getting a hard push from a growing number of transportation experts and officials. That is because the traditional by-the-gallon fuel tax, struggling to keep up with road building and maintenance demands, could fall even farther behind as vehicles’ gas mileage rises and more alternative-fuel vehicles come on line.

The idea of shifting to a by-the-mile tax has been discussed for years, but it now appears to be getting more serious attention. A federal commission, after a two-year study, concluded earlier this year that the road tax was the “best path forward” to keep revenues flowing to highway and transportation projects, and could be an important new tool to help manage traffic and relieve congestion.